1. Field of the Invention
The present invention relates to radio communication systems and its control methods and more particularly, to a radio communication system which includes a plurality of base units and at least one radio mobile terminal to execute the connecting operation of an incoming-signal from any one of the plurality of base units to the radio mobile terminal and a method for controlling the radio communication system.
2. Description of the Related Art
A conventional radio communication system of the type referred to is arranged as shown in FIG. 26. In the drawing, a radio control station 101 is connected at its one side with a wired telephone network 102 and also at the other side with radio base stations 103-1, 103-2 and 103-3 through their wired lines. The radio base stations 103-1, 103-2 and 103-3 have radio zones 104-1, 104-2 and 104-3 respectively. When a portable radio telephone set 105 as a mobile station is located in the radio zone 104-2, the portable radio telephone set 105 may be coupled with the radio base station 103-2 through a radio link, in which case the radio telephone set 105 can communicate with the wired telephone network 102 through the radio control station 101 and the radio base station 103-2.
When the portable radio telephone set 105 issues a calling signal, the calling signal transmitted from the telephone set 105 is received at any of the radio base stations 103-1 to 103-3, even when the radio telephone set 105 is located in any of the radio zones 104-1 to 104-3, so that an interconnection control between the portable radio telephone set 105 and the wired telephone network 102 is realized to start a communication.
On the other hand, when an incoming signal directed to the portable radio telephone set 105 is sent from the wired telephone network 102, the system must inform the radio telephone set 105 of the presence of the incoming signal.
To this end, in the conventional system, the radio base stations 103-1 to 103-3 transmit the incoming signal respectively under control of the radio control station 101, whereby the portable radio telephone set 105 can receive the incoming signal even when the telephone set 105 is located in any of the radio zones 104-1 to 104-3. Therefore, the base stations 103-1 to 103-3 must use their control channels for the transmission of the incoming signal. Since the incoming signal is transmitted even to the radio zones where the portable radio telephone set 105 is not present, the system performs useless data communication. When each radio zone is small and a large number of such radio zones exist, a large quantity of useless data communication must be performed, which is undesirable from the viewpoint of effective use of electromagnetic waves.
For the purpose of avoiding the above disadvantage, there has been proposed such a method that the location of the portable radio telephone set 105 belonging to one of the radio zones 104-1 to 104-3 is previously registered in the radio control station 101 so that, when the radio control station 101 receives an incoming signal, the control station controls the radio base station of the registered radio zone to transmit the incoming signal to the corresponding portable radio telephone set 105, for example, as disclosed in U.S. patent application Ser. No. 07/408249. In this case, the location of the portable radio telephone set 105 is registered by intermittent transmission of a registration signal from the portable telephone set 105 in its wait or ready state to the radio control station 101 through its control channel to inform the control station 101 of the current location of the portable telephone set 105.
However, since the control channel is frequently used, this inevitably requires many control channels for traffic processing, thus disturbing the normal signal transmitting of the other portable telephone sets (not shown). In addition, since the registration requires the repetitive transmission of the registration signal from the portable radio telephone set, the battery of the portable telephone set as its power source is undesirably exhausted.
As discussed above, in the conventional radio communication system, when an incoming signal is received, the incoming signal is transmitted from the plurality of radio base stations through their different control channels. Alternatively, a signal for registering the radio zone to which the incoming signal is to be transmitted is intermittently transmitted from each portable radio telephone set. As a result, the traffic quantity of the control channel is increased and thus a large number of control channels are required. Thus, radio channels by electromagnetic waves cannot effectively be used. Further, due to the intermittent transmission of the signal for registration of the radio zone from each portable radio telephone set, the battery of the portable radio telephone set is greatly consumed.